Gene therapy of central nervous system (CNS) disorders poses several unique obstacles, among them gene delivery across the blood brain barrier. Recent observations suggest that some CNS glial cells may derive from macrophages. If macrophages do indeed colonize and repopulate the CNS in significant numbers, they represent a potential route for gene delivery to the CNS. We will test the hypothesis that CNS glial cells are derived from hematopoietic progenitor cells. Hematopoietic stem cells from C3H mice will be infected with recombinant retroviruses that express the lac Z gene; expression of this gene can be detected in situ using simple enzymic histochemical stains. Genetically modified stems cells will be injected into lethally irradiated syngeneic mice, and the mice will be monitored for engraftment by analyzing peripheral blood of transplant recipients for lac Z expression. Animals that express lac Z in hematopoietic lineages will be euthanized at various times following the initial transplant and analyzed for lac Z expression in cellular components of the CNS by enzymic histochemistry. Similar experiments will be performed using retroviral vectors containing the human HPRT gene and HPRT-deficient mice. These studies should provide the initial data necessary to model macrophage migration into and colonization of the CNS. Such models are the first step in the determination of the feasibility of macrophage-mediated gene delivery into the CNS.